In the recent electronic parts mounting technology, products using electronic circuit boards mounting electronic parts are increasing in number. In this background, there is a mounting demand for enhancement of mechanical junction strength of soldered parts, increase of thermal impact strength, and higher reliability. On the other hand, while the concern about the global environmental protection is attracting attention, statutory restrictions are being considered about disposal of industrial refuse such as electronic circuit boards.
An outline of solder alloy of electrode surface for solder junction of electronic parts and its soldering method of a prior art is described below while referring to drawings. FIG. 1 is an outline drawing showing an electrode composition of a conventional electrode for joining electronic parts, and FIG. 2 is a metal composition diagram of the junction interface of a conventional electrode for joining electronic parts.
In FIG. 1, reference numeral 3 is Sn--Pb, 4 is Ni, and 5 is Ag, and they are solder alloys, and compose an electrode of an electronic part 6. In particular, as the electrode surface material, an alloy of Sn and Pb is used. In the metal composition diagram of the junction interface of the electrode for joining electronic parts in FIG. 2, reference numeral 1 is an .alpha.-solid solution, being in Sn-rich phase. Reference numeral 2 is a .beta.-solid solution, being in Pb-rich phase. In such conventional solder alloy, the metal composition on the electrode surface is composed of 90 wt. % of Sn and 10 wt. % of Pb, and its melting point is 183.degree. C. to 210.degree. C. When soldering such electrode with a solder alloy of metal for junction, its alloy composition is a lamellar form of .alpha.-solid solution 1 and .beta.-solid solution 2.
In conventional soldering, the solder alloy of electrode for solder junction of electronic parts had such problems. When soldered by the solder alloy for junction, the alloy composition became lamellar. In particular, when exposed repeatedly to high temperature environments, the texture was swollen, and hence when a stress is applied to the solder, slipping occurred on the texture interface, which resulted in solder cracks.
Or, from the viewpoint of environmental protection, the regulation of lead contained in the solder alloy (Sn--Pb alloy) is being promoted, and when the refuse of electronic circuit board soldered by the conventional solder alloy is exposed to acid rain, lead is eluted massively, and this eluate is known to have an adverse effect on the human health.
In the light of the above problems, it is an object of the invention to present an electrode surface constitution of an electronic part by a solder alloy, free from lead in the solder junction area, in soldering of electronic parts, ultra-fine in the alloy texture of the junction area, and excellent in repeated thermal fatigue characteristic in high temperature environments.